ErosionFrom Wikipedia, the free encyclopediaJump to: navigation, searchFor morphological image processing operations, see Erosion (morphology). For use of indermatopathology, see Erosion (dermatopathology). This article uses first-person ("I"; "we") or second-person ("you") inappropriately. Please rewrite it to use a more formal, encyclopedic tone. (January 2013)A natural arch produced by the erosion of differentially weathered rock in Jebel Kharaz,JordanErosion is the process by which soil and rock are removed from the Earths surface byexogenetic processes such as wind or water flow, and then transported and deposited inother locations.While erosion is a natural process, human activities have increased by 10-40 times therate at which erosion is occurring globally. Excessive erosion causes problems such asdesertification, decreases in agricultural productivity due to land degradation,sedimentation of waterways, and ecological collapse due to loss of the nutrient rich uppersoil layers. Water and wind erosion are now the two primary causes of land degradation;combined, they are responsible for 84% of degraded acreage, making excessive erosionone of the most significant global environmental problems.Industrial agriculture, deforestation, roads, anthropogenic climate change and urbansprawl are amongst the most significant human activities in regard to their effect onstimulating erosion. However, there are many available alternative land use practicesthat can curtail or limit erosion, such as terrace-building, no-till agriculture, andrevegetation of denuded soils.Contents
• 1 Physical processes o 1.1 Water erosion 1.1.1 Rainfall 1.1.2 Rivers and streams 1.1.3 Coastal erosion 1.1.4 Glaciers 1.1.5 Floods 1.1.6 Freezing and thawing o 1.2 Wind erosion o 1.3 Gravitational erosion o 1.4 Exfoliation • 2 Factors affecting erosion rates o 2.1 Precipitation and wind speed o 2.2 Soil structure and composition o 2.3 Vegetative cover o 2.4 Topography • 3 Human activities that increase erosion rates o 3.1 Agricultural practices o 3.2 Deforestation o 3.3 Roads and urbanization o 3.4 Climate change • 4 Global environmental effects o 4.1 Land degradation o 4.2 Sedimentation of aquatic ecosystems o 4.3 Airborne dust pollution o 4.4 Tectonic effects • 5 Monitoring, measuring, and modeling erosion • 6 Prevention and remediation • 7 See also • 8 Notes • 9 Further reading • 10 External linksPhysical processesWater erosionRainfall
A hillside covered in rills and gullies due to erosion processes caused by rainfallThere are three primary types of erosion that occur as a direct result of rainfall—sheeterosion, rill erosion, and gully erosion. Sheet erosion is generally seen as the first andleast severe stage in the soil erosion process, which is followed by rill erosion, and finallygully erosion (the most severe of the three).The impact of a falling raindrop creates a small crater in the soil, ejecting soil particles.The distance these soil particles travel can be as much as two feet vertically and five feethorizontally on level ground. Once the rate of rainfall is faster than the rate of infiltrationinto the soil, surface runoff occurs and carries the loosened soil particles down the slope.Sheet erosion is the transport of loosened soil particles by overland flow.Rill erosion refers to the development of small, ephemeral concentrated flow paths whichfunction as both sediment source and sediment delivery systems for erosion on hillslopes.Generally, where water erosion rates on disturbed upland areas are greatest, rills areactive. Flow depths in rills are typically on the order of a few centimeters or less andslopes may be quite steep. This means that rills exhibit very different hydraulic physicsthan water flowing through the deeper, wider channels of streams and rivers.Gully erosion occurs when runoff water accumulates, and then rapidly flows in narrowchannels during or immediately after heavy rains or melting snow, removing soil to aconsiderable depth.Rivers and streamsFor more details on waters erosive ability, see Hydraulic action.
Dobbingstone Burn, Scotland—This photo illustrates two different types of erosionaffecting the same place. Valley erosion is occurring due to the flow of the stream, andthe boulders and stones (and much of the soil) that are lying on the edges are glacial tillthat was left behind as ice age glaciers flowed over the terrain.Valley or stream erosion occurs with continued water flow along a linear feature. Theerosion is both downward, deepening the valley, and headward, extending the valley intothe hillside. In the earliest stage of stream erosion, the erosive activity is dominantlyvertical, the valleys have a typical V cross-section and the stream gradient is relativelysteep. When some base level is reached, the erosive activity switches to lateral erosion,which widens the valley floor and creates a narrow floodplain. The stream gradientbecomes nearly flat, and lateral deposition of sediments becomes important as the streammeanders across the valley floor. In all stages of stream erosion, by far the most erosionoccurs during times of flood, when more and faster-moving water is available to carry alarger sediment load. In such processes, it is not the water alone that erodes: suspendedabrasive particles, pebbles and boulders can also act erosively as they traverse a surface,in a process known as traction.Bank erosion is the wearing away of the banks of a stream or river. This is distinguishedfrom changes on the bed of the watercourse, which is referred to as scour. Erosion andchanges in the form of river banks may be measured by inserting metal rods into the bankand marking the position of the bank surface along the rods at different times.Thermal erosion is the result of melting and weakening permafrost due to moving water. It can occur both along rivers and at the coast. Rapid river channel migration observedin the Lena River of Siberia is due to thermal erosion, as these portions of the banks arecomposed of permafrost-cemented non-cohesive materials. Much of this erosionoccurs as the weakened banks fail in large slumps. Thermal erosion also affects theArctic coast, where wave action and near-shore temperatures combine to undercutpermafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a100-kilometer segment of the Beaufort Sea shoreline averaged 5.6 meters per year from1955 to 2002.Coastal erosionMain article: Coastal erosion
See also: Beach evolutionWave cut platform caused by erosion of cliffs by the sea, at Southerndown in SouthWalesShoreline erosion, which occurs on both exposed and sheltered coasts, primarily occursthrough the action of currents and waves but sea level (tidal) change can also play a role.Hydraulic action takes place when air in a joint is suddenly compressed by a waveclosing the entrance of the joint. This then cracks it. Wave pounding is when the sheerenergy of the wave hitting the cliff or rock breaks pieces off. Abrasion or corrasion iscaused by waves launching seaload at the cliff. It is the most effective and rapid form ofshoreline erosion (not to be confused with corrosion). Corrosion is the dissolving of rockby carbonic acid in sea water. Limestone cliffs are particularly vulnerable to this kind oferosion. Attrition is where particles/seaload carried by the waves are worn down as theyhit each other and the cliffs. This then makes the material easier to wash away. Thematerial ends up as shingle and sand. Another significant source of erosion, particularlyon carbonate coastlines, is the boring, scraping and grinding of organisms, a processtermed bioerosion.Sediment is transported along the coast in the direction of the prevailing current(longshore drift). When the upcurrent amount of sediment is less than the amount beingcarried away, erosion occurs. When the upcurrent amount of sediment is greater, sand orgravel banks will tend to form as a result of deposition. These banks may slowly migratealong the coast in the direction of the longshore drift, alternately protecting and exposingparts of the coastline. Where there is a bend in the coastline, quite often a build up oferoded material occurs forming a long narrow bank (a spit). Armoured beaches andsubmerged offshore sandbanks may also protect parts of a coastline from erosion. Overthe years, as the shoals gradually shift, the erosion may be redirected to attack differentparts of the shore.Glaciers
Glacial moraines above Lake Louise, in Alberta, Canada.Glaciers erode predominantly by three different processes: abrasion/scouring, plucking,and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed,polishing and gouging the underlying rocks, similar to sandpaper on wood. Glaciers canalso cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, theglacier freezes to its bed, then as it surges forward, it moves large sheets of frozensediment at the base along with the glacier. This method produced some of the manythousands of lake basins that dot the edge of the Canadian Shield. These processes,combined with erosion and transport by the water network beneath the glacier, leavemoraines, drumlins, ground moraine (till), kames, kame deltas, moulins, and glacialerratics in their wake, typically at the terminus or during glacier retreat.FloodsAt extremely high flows, kolks, or vortices are formed by large volumes of rapidlyrushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called Rock-cut basins. Examples can be seen in the floodregions result from glacial Lake Missoula, which created the channeled scablands in theColumbia Basin region of eastern Washington.Freezing and thawingCold weather causes water trapped in tiny rock cracks to freeze and expand, breaking therock into several pieces. This can lead to gravity erosion on steep slopes. The scree whichforms at the bottom of a steep mountainside is mostly formed from pieces of rock (soil)broken away by this means. It is a common engineering problem wherever rock cliffs arealongside roads, because morning thaws can drop hazardous rock pieces onto the road.Wind erosion
Árbol de Piedra, a rock formation in the Altiplano, Bolivia sculpted by wind erosion.Main article: Aeolian processesWind erosion is a major geomorphological force, especially in arid and semi-arid regions.It is also a major source of land degradation, evaporation, desertification, harmfulairborne dust, and crop damage—especially after being increased far above natural ratesby human activities such as deforestation, urbanization, and agriculture.Wind erosion is of two primary varieties: deflation, where the wind picks up and carriesloose soil particles; and abrasion, where surfaces are worn down as they are struck byairborne particles carried by wind. Deflation is divided into three categories: (1) surfacecreep, where larger, heavier particles slide or roll along the ground; (2) saltation, whereparticles are lifted a short height into the air, and bounce and saltate across the surface ofthe soil; and (3) suspension, where very small and light particles are lifted into the air bythe wind, and are often carried for long distances. Saltation is responsible for the majority(50-70%) of wind erosion, followed by suspension (30-40%), and then surface creep (5-25%).Wind erosion is much more severe in arid areas, and during times of drought. Forexample, in the Great Plains, it is estimated that wind erosion soil loss can be as much as6100 times greater in drought years, than in wet years.Gravitational erosionWadi in Makhtesh Ramon, Israel, showing gravity collapse erosion on its banks.
Mass movement is the downward and outward movement of rock and sediments on asloped surface, mainly due to the force of gravity.Mass movement is an important part of the erosional process, and is often the first stagein the breakdown and transport of weathered materials in mountainous areas. It movesmaterial from higher elevations to lower elevations where other eroding agents such asstreams and glaciers can then pick up the material and move it to even lower elevations.Mass-movement processes are always occurring continuously on all slopes; some mass-movement processes act very slowly; others occur very suddenly, often with disastrousresults. Any perceptible down-slope movement of rock or sediment is often referred to ingeneral terms as a landslide. However, landslides can be classified in a much moredetailed way that reflects the mechanisms responsible for the movement and the velocityat which the movement occurs. One of the visible topographical manifestations of a veryslow form of such activity is a scree slope.Slumping happens on steep hillsides, occurring along distinct fracture zones, often withinmaterials like clay that, once released, may move quite rapidly downhill. They will oftenshow a spoon-shaped isostatic depression, in which the material has begun to slidedownhill. In some cases, the slump is caused by water beneath the slope weakening it. Inmany cases it is simply the result of poor engineering along highways where it is aregular occurrence.Surface creep is the slow movement of soil and rock debris by gravity which is usuallynot perceptible except through extended observation. However, the term can alsodescribe the rolling of dislodged soil particles 0.5 to 1.0 mm in diameter by wind alongthe soil surface.ExfoliationExfoliation is a type of erosion that occurs when a rock is rapidly heated up by the sun.This results in the expansion of the rock. When the temperature decreases again, the rockcontracts, causing pieces of the rock to break off. Exfoliation occurs mainly in desertsdue to the high temperatures during the day and cold temperatures at night.Factors affecting erosion ratesPrecipitation and wind speedClimatic factors include the amount and intensity of precipitation, the averagetemperature, as well as the typical temperature range, seasonality, wind speed, and stormfrequency. In general, given similar vegetation and ecosystems, areas with high-intensityprecipitation, more frequent rainfall, more wind, or more storms are expected to havemore erosion.Rainfall intensity is the primary determinant of erosivity, with higher intensity rainfallgenerally resulting in more erosion. The size and velocity of rain drops is also an
important factor. Larger and higher-velocity rain drops have greater kinetic energy, andthus their impact will displace soil particles by larger distances than smaller, slower-moving rain drops.Soil structure and compositionErosional gully in unconsolidated Dead Sea (Israel) sediments along the southwesternshore. This gully was excavated by floods from the Judean Mountains in less than a year.The composition, moisture, and compaction of soil are all major factors in determiningthe erosivity of rainfall. Sediments containing more clay tend to be more resistant toerosion than those with sand or silt, because the clay helps bind soil particles together.Soil containing high levels of organic materials are often more resistant to erosion,because the organic materials coagulate soil colloids and create a stronger, more stablesoil structure. The amount of water present in the soil before the precipitation alsoplays an important role, because it sets limits on the amount of water that can be absorbedby the soil (and hence prevented from flowing on the surface as erosive runoff). Wet,saturated soils will not be able to absorb as much rain water, leading to higher levels ofsurface runoff and thus higher erosivity for a given volume of rainfall. Soilcompaction also affects the permeability of the soil to water, and hence the amount ofwater that flows away as runoff. More compacted soils will have a larger amount ofsurface runoff than less compacted soils.Vegetative coverSee also: Vegetation and slope stabilityVegetation acts as an interface between the atmosphere and the soil. It increases thepermeability of the soil to rainwater, thus decreasing runoff. It shelters the soil fromwinds, which results in decreased wind erosion, as well as advantageous changes inmicroclimate. The roots of the plants bind the soil together, and interweave with otherroots, forming a more solid mass that is less susceptible to both water and wind erosion.The removal of vegetation increases the rate of surface erosion.Topography
The topography of the land determines the velocity at which surface runoff will flow,which in turn determines the erosivity of the runoff. Longer, steeper slopes (especiallythose without adequate vegetative cover) are more susceptible to very high rates oferosion during heavy rains than shorter, less steep slopes. Steeper terrain is also moreprone to mudslides, landslides, and other forms of gravitational erosion processes.Human activities that increase erosion ratesAgricultural practicesTilled farmland such as this is very susceptible to erosion from rainfall, due to thedestruction of vegetative cover and the loosening of the soil during plowing.Unsustainable agricultural practices are the single greatest contributor to the globalincrease in erosion rates. The tillage of agricultural lands, which breaks up soil intofiner particles, is one of the primary factors. The problem has been exacerbated inmodern times, due to mechanized agricultural equipment that allows for deep plowing,which severely increases the amount of soil that is available for transport by watererosion. Others include mono-cropping, farming on steep slopes, pesticide and chemicalfertilizer usage (which kill organisms that bind soil together), row-cropping, and the useof surface irrigation. A complex overall situation with respect to defining nutrientlosses from soils, could arise as a result of the size selective nature of soil erosion events.Loss of total phosphorus, for instance, in the finer eroded fraction is greater relative to thewhole soil.  Extrapolating this evidence to predict subsequent behaviour withinreceiving aquatic systems, the reason is that this more easily transported material maysupport a lower solution P concentration compared to coarser sized fractions.  Tillagealso increases wind erosion rates, by dehydrating the soil and breaking it up into smallerparticles that can be picked up by the wind. Exacerbating this is the fact that most of thetrees are generally removed from agricultural fields, allowing winds to have long, openruns to travel over at higher speeds. Heavy grazing reduces vegetative cover andcauses severe soil compaction, both of which increase erosion rates.Deforestation
In this clearcut, almost all of the vegetation has been stripped from surface of steepslopes, in an area with very heavy rains. Severe erosion occurs in cases such as this,causing stream sedimentation and the loss of nutrient rich topsoil.In an undisturbed forest, the mineral soil is protected by a layer of leaf litter and anhumus that cover the forest floor. These two layers form a protective mat over the soilthat absorbs the impact of rain drops. They are porous and highly permeable to rainfall,and allow rainwater to slow percolate into the soil below, instead of flowing over thesurface as runoff. The roots of the trees and plants hold together soil particles,preventing them from being washed away. The vegetative cover acts to reduce thevelocity of the raindrops that strike the foliage and stems before hitting the ground,reducing their kinetic energy. However it is the forest floor, more than the canopy, thatprevents surface erosion. The terminal velocity of rain drops is reached in about 8 meters.Because forest canopies are usually higher than this, rain drops can often regain terminalvelocity even after striking the canopy. However, the intact forest floor, with its layers ofleaf litter and organic matter, is still able to absorb the impact of the rainfall.Deforestation causes increased erosion rates due to exposure of mineral soil by removingthe humus and litter layers from the soil surface, removing the vegetative cover that bindssoil together, and causing heavy soil compaction from logging equipment. Once treeshave been removed by fire or logging, infiltration rates become high and erosion low tothe degree the forest floor remains intact. Severe fires can lead to significant furthererosion if followed by heavy rainfall.Globally one of the largest contributors to erosive soil loss in the year 2006 is the slashand burn treatment of tropical forests. In a number of regions of the earth, entire sectorsof a country have been rendered unproductive. For example, on the Madagascar highcentral plateau, comprising approximately ten percent of that countrys land area,virtually the entire landscape is sterile of vegetation, with gully erosive furrows typicallyin excess of 50 meters deep and one kilometer wide. Shifting cultivation is a farmingsystem which sometimes incorporates the slash and burn method in some regions of theworld. This degrades the soil and causes the soil to become less and less fertile.Roads and urbanizationUrbanization has major effects on erosion processes—first by denuding the land ofvegetative cover, altering drainage patterns, and compacting the soil during construction;and next by covering the land in an impermeable layer of asphalt or concrete thatincreases the amount of surface runoff and increases surface wind speeds. Much of thesediment carried in runoff from urban areas (especially roads) is highly contaminatedwith fuel, oil, and other chemicals. This increased runoff, in addition to eroding anddegrading the land that it flows over, also causes major disruption to surroundingwatersheds by altering the volume and rate of water that flows through them, and fillingthem with chemically polluted sedimentation. The increased flow of water through localwaterways also causes a large increase in the rate of bank erosion.
Climate changeMain article: Land degradationThe warmer atmospheric temperatures observed over the past decades are expected tolead to a more vigorous hydrological cycle, including more extreme rainfall events.The rise in sea levels that has occurred as a result of climate change has also greatlyincreased coastal erosion rates.Studies on soil erosion suggest that increased rainfall amounts and intensities will lead togreater rates of erosion. Thus, if rainfall amounts and intensities increase in many parts ofthe world as expected, erosion will also increase, unless amelioration measures are taken.Soil erosion rates are expected to change in response to changes in climate for a varietyof reasons. The most direct is the change in the erosive power of rainfall. Other reasonsinclude: a) changes in plant canopy caused by shifts in plant biomass productionassociated with moisture regime; b) changes in litter cover on the ground caused bychanges in both plant residue decomposition rates driven by temperature and moisturedependent soil microbial activity as well as plant biomass production rates; c) changes insoil moisture due to shifting precipitation regimes and evapo-transpiration rates, whichchanges infiltration and runoff ratios; d) soil erodibility changes due to decrease in soilorganic matter concentrations in soils that lead to a soil structure that is more susceptibleto erosion and increased runoff due to increased soil surface sealing and crusting; e) ashift of winter precipitation from non-erosive snow to erosive rainfall due to increasingwinter temperatures; f) melting of permafrost, which induces an erodible soil state from apreviously non-erodible one; and g) shifts in land use made necessary to accommodatenew climatic regimes.Studies by Pruski and Nearing indicated that, other factors such as land use notconsidered, we can expect approximately a 1.7% change in soil erosion for each 1%change in total precipitation under climate change.Global environmental effectsWorld map indicating areas that are vulnerable to high rates of water erosion.
During the 17th and 18th centuries, Easter Island experienced severe erosion due todeforestation and unsustainable agricultural practices. The resulting loss of topsoilultimately led to ecological collapse, causing mass starvation and the completedisintegration of the Easter Island civilization.Due to the severity of its ecological effects, and the scale on which it is occurring, erosionconstitutes one of the most significant global environmental problems we face today. Land degradationWater and wind erosion are now the two primary causes of land degradation; combined,they are responsible for 84% of degraded acreage.Each year, about 75 billion tons of soil is eroded from the land—a rate that is about 13-40times as fast as the natural rate of erosion. Approximately 40% of the worldsagricultural land is seriously degraded. According to the United Nations, an area offertile soil the size of Ukraine is lost every year because of drought, deforestation andclimate change. In Africa, if current trends of soil degradation continue, the continentmight be able to feed just 25% of its population by 2025, according to UNUs Ghana-based Institute for Natural Resources in Africa.The loss of soil fertility due to erosion is further problematic because the response isoften to apply chemical fertilizers, which leads to further water and soil pollution, ratherthan to allow the land to regenerate.Sedimentation of aquatic ecosystemsSoil erosion (especially from agricultural activity) is considered to be the leading globalcause of diffuse water pollution, due to the effects of the excess sediments flowing intothe worlds waterways. The sediments themselves act as pollutants, as well as beingcarriers for other pollutants, such as attached pesticide molecules or heavy metals.The effect of increased sediments loads on aquatic ecosystems can be catastrophic. Siltcan smother the spawning beds of fish, by filling in the space between gravel on thestream bed. It also reduces their food supply, and causes major respiratory issues for them
as sediment enters their gills. The biodiversity of aquatic plant and algal life is reduced,and invertebrates are also unable to survive and reproduce. While the sedimentation eventitself might be relatively short-lived, the ecological disruption caused by the mass die offoften persists long into the future.One of the most serious and long-running water erosion problems worldwide is in thePeoples Republic of China, on the middle reaches of the Yellow River and the upperreaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sedimentflows into the ocean each year. The sediment originates primarily from water erosion inthe Loess Plateau region of the northwest.Airborne dust pollutionSoil particles picked up during wind erosion are a major source of air pollution, in theform of airborne particulates—"dust". These airborne soil particles are oftencontaminated with toxic chemicals such as pesticides or petroleum fuels, posingecological and public health hazards when they later land, or are inhaled/ingested.Dust from erosion acts to suppress rainfall and changes the sky color from blue to white,which leads to an increase in red sunsets. Over 50% of the African dust that reaches theUnited States affects Florida. Dust events have been linked to a decline in the health ofcoral reefs across the Caribbean and Florida, primarily since the 1970s. Similar dustplumes originate in the Gobi desert, which combined with pollutants, spread largedistances downwind, or eastward, into North America.Tectonic effects This section requires expansion. (April 2012)Main article: Erosion and tectonicsThe removal by erosion of large amounts of rock from a particular region, and itsdeposition elsewhere, can result in a lightening of the load on the lower crust and mantle.This can cause tectonic or isostatic uplift in the region.Monitoring, measuring, and modeling erosion
Terracing is an ancient technique that can significantly slow the rate of water erosion oncultivated slopes.See also: Erosion prediction This section requires expansion. (April 2012)Monitoring and modeling of erosion processes can help us better understand the causes,make predictions, and plan how to implement preventative and restorative strategies.However, the complexity of erosion processes and the number of areas that must bestudied to understand and model them (e.g. climatology, hydrology, geology, chemistry,physics, etc.) makes accurate modelling quite challenging. Erosion models are alsonon-linear, which makes them difficult to work with numerically, and makes it difficultor impossible to scale up to making predictions about large areas from data collected bysampling smaller plots.The most commonly used model for predicting soil loss from water erosion is theUniversal Soil Loss Equation (USLE), which estimates the average annual soil loss as:where R is the rainfall erosivity factor, K is the soil erodibility factor, L and S aretopographic factors representing length and slope, and C and P are cropping managementfactors.Erosion is measured and further understood using tools such as the micro-erosion meter(MEM) and the traversing micro-erosion meter (TMEM). The MEM has proved helpfulin measuring bedrock erosion in various ecosystems around the world. It can measureboth terrestrial and oceanic erosion. On the other hand, the TMEM can be used to trackthe expanding and contracting of volatile rock formations and can give a reading of howquickly a rock formation is deteriorating.Prevention and remediation
See also: Erosion controlA windbreak (the row of trees) planted next to an agricultural field, acting as a shieldagainst strong winds. This reduces the effects of wind erosion, and provides many otherbenefits.The most effective known method for erosion prevention is to increase vegetative coveron the land, which helps prevent both wind and water erosion. Terracing is anextremely effective means of erosion control, which has been practiced for thousands ofyears by people all over the world. Windbreaks (also called shelterbelts) are rows oftrees and shrubs that are planted along the edges of agricultural fields, to shield the fieldsagainst winds. In addition to significantly reducing wind erosion, windbreaks providemany other benefits such as improved microclimates for crops (which are sheltered fromthe dehydrating and otherwise damaging effects of wind), habitat for beneficial birdspecies, carbon sequestration, and aesthetic improvements to the agriculturallandscape. Traditional planting methods, such as mixed-cropping (instead ofmonocropping) and crop rotation have also been shown to significantly reduce erosionrates.